Insoles have long been used to cushion the impact of a person's foot with a supporting surface. The basic concept of shock absorption by, for example, use of a fluid-containing insole that transfers the fluid from the heel to the ball or forward portion of the user's foot has been known for many years. Furthermore, gas-filled sacs have also been used as orthopedic cushions to replace the inner sole of shoes. To this end, gas cushions have been designed to accomplish a number of varied purposes, including to provide an orthopedically shaped surface generally conforming to the curvature of the human foot, to provide means for softening the jar incident to the operation of walking, to provide means concealed within the shoe for increasing the apparent height of a human being, to correct a manner of walking of the wearer, and to form a comfortable and natural support for the arch of the foot.
Inflated cushioning and impact-absorbing insole devices adapted for placement within articles of footwear are also known. Such devices generally have an upper layer and lower layer formed of a flexible, fluid-impermeable material, and may be filled with a fluid or gel. The upper and lower layers are sealed together, thereby forming a plurality of sealed, laterally-spaced tubular members. The volume of fluid or gel disposed within the tubular chambers may entirely or partially fill all of the chambers. Existing liquid or gels used for this purpose include those disclosed in U.S. Pat. No. 4,756,311; such as a mixture of carbopol.TM., propyleneglycol, formaldehyde, sodium hydroxide, color dye and water. Another fluid or gel disclosed for this purpose is that of U.S. Pat. No. 3,885,403, which comprises water, glycol and carbopol 940.
However, a significant disadvantage associated with existing fluid or gel-filled insoles is that the fluid or gel breaks down as a result of repeated use, thus significantly reducing the cushioning ability of the insole. Accordingly, there is a need in the art for a material which, when used as a component for cushioning insoles, resists breakdown over the course of normal use. The present invention fulfills this need, and provides further related advantages.